1. Field of the invention
This invention relates to an automatically controlled feed forward nonlinear distortion compensation amplifier (which will be hereinafter referred to as an FF amplifier) and an improvement in a control circuit applied thereto.
2. Description of the related art
An automatically controlled feed forward nonlinear distortion compensation system is one of many techniques for providing amplifiers with very low distortion. An amplifier provided with this system, that is, an FF amplifier, consists of a loop for detecting distortions and a loop for eliminating them.
FIG. 3 shows a configuration example of the conventional FF amplifier wherein a distortion detection loop 10 consists of a divider 12, a variable attenuator 14, a variable phase shifter 16, a main amplifier 18, and a directional coupler 20. A distortion elimination loop 22 consists of a variable attenuator 24, a variable phase shifter 26, an auxiliary amplifier 28, and a directional coupler 30. Further, a control circuit 32 is provided to control the variable attenuators 14 and 24 and the variable phase shifters 16 and 26.
The distortion detection loop 10 operates so as to extract only distortion elements occurring in the main amplifier 18. The distortion elimination loop 22 operates so as to add the distortion elements extracted by the distortion detection loop 10 to the output of the main amplifier 18 but in the opposite phase for eliminating the distortion elements. First, input to the FF amplifier is supplied by the divider 12 to the variable attenuator 14 and the directional coupler 20. The variable attenuator 14 attenuates the input signals from the divider 12 and the variable phase shifter 16 shifts the phase of the signals attenuated by the variable attenuator 14. The main amplifier 18 amplifies the output of the variable phase shifter 16, then outputs the amplified signals via the directional couplers 20 and 30 to an external device. At the same time, the distortion elements produced in the main amplifier 18 are extracted by the directional coupler 20 in response to the output of the main amplifier 18 and the signal supplied from the divider 12 for feeding into the variable attenuator 24. To optimize extraction of the distortion element, "a first optimum operation condition" must be satisfied, as described below.
The variable attenuator 24 attenuates the distortion elements fed from the directional coupler 20 and the variable phase shifter 26 shifts the phase of the attenuated distortion elements. The auxiliary amplifier 28 amplifies the output of the variable phase shifter 26. The output of the auxiliary amplifier 28 is added by the directional coupler 30 to the output of the directional coupler 20. Thus, the distortion elements are added to the signals amplified by the main amplifier 18 in opposite phase, thereby eliminating the distortion elements from the output of the FF amplifier if "a second optimum operation condition" described below is satisfied.
To optimize operation of an FF amplifier having such a configuration, the following two optimum operation conditions must be satisfied: The first optimum operation condition requires that the transfer function of the path denoted by 100 in FIG. 3 be equivalent to the transfer function of the path denoted by 110 except that they have opposite signs and similarly the second requires that the transfer function of the path denoted by 200 in FIG. 3 be equivalent to the transfer function of the path denoted by 210 except that they have opposite signs.
The variable attenuators 14 and 24 and the variable phase shifters 16 and 26 are provided to satisfy these optimum operation conditions. The control circuit 32 is provided to control the attenuators and phase shifters to enable the FF amplifier to always operate under the optimum operation conditions.
For the FF amplifier to make distortion compensation for providing the improvement effect of 30 dB or more, the gain variation and phase variation in each of the loops 10 and 22 must be kept within .+-.0.3 dB and .+-.2.degree. respectively. Thus, very flat frequency characteristics are required for the parts making up the FF amplifier. On the other hand, the variable attenuators 14 and 24 and the variable phase shifters 16 and 26 which can cause the amounts of attenuation and phase shift to be continuously changed are required for the FF amplifier to make distortion compensation, configuration examples of the variable attenuators and variable phase shifters being as shown in FIGS. 4 and 5 respectively.
The variable attenuator shown in FIG. 4 is a circuit using a PIN diode 34. For example, signal inputs from the divider 12 and control inputs from the control circuit 32 are applied to the PIN diode 34. The variable phase shifter shown in FIG. 5 is provided with a variable capacitance diode 36 and a circulator 38; the signal inputs are applied to the circulator 38 and the control inputs to the cathode of the variable capacitance diode 36.
As described above, very flat frequency characteristics of gain and phase shift amount are required for the parts making up the FF amplifier. Amplification circuits with semiconductors require some temperature compensation since their characteristics change in response to a temperature change because of the nature of semiconductor material. However, conventional temperature compensation methods have several problems.
The following temperature compensation methods are available: Use of a thermosensitive device to provide a bias circuit for transistors making up an amplification circuit; or location of a temperature compensation circuit separate from an amplification circuit. If complete temperature compensation is enabled by these methods, complete distortion compensation operation will always be performed even when the amounts of attenuation and phase shift cannot be automatically controlled. Resultantly, leading-in operation, when automatic control is started, becomes fast and control operation also becomes stable. However, it is very difficult to decide temperature compensation circuit constants.
As an alternate temperature compensation method, the control ranges in the variable attenuators 14 and 24 and the variable phase shifters 16 and 26 (the ranges of the amounts of attenuation and phase shift that can be controlled) are preset widely and then changes in gains and the phase shift amount are absorbed under the control of the variable attenuators 14 and 24 and the variable phase shifters 16 and 26. This method is preferred to the method using a temperature compensation circuit or the like in that the need for the difficult decision of circuit constants is eliminated; however, distortion compensation operation without controlling gains or phases, that is, in a free running condition is not always placed in the best condition. Therefore, leading-in operation, when automatic control is started, becomes slow and stability of control operation gets worse. When a digital-to-analog converter is used at the output stage of the control circuit 32, if the control range is set widely, resolution per bit is degraded.
The conventional FF amplifier also contains some problems with adjustments. To adjust the FF amplifier for starting distortion elimination operation, if the distortion detection loop and distortion elimination loop are adjusted so that the FF amplifier performs the best distortion compensation operation in a free running condition with no automatic control, then the leading-in operation, when automatic control is started, becomes fast and control operation also becomes stable. For this purpose, however, it is necessary to provide a variable attenuator and a variable phase shifter separate from the variable attenuators 14 and 24 and the variable phase shifters 16 and 26 for automatic control or it is necessary to adjust the gains and phase shift amounts of the main amplifier 18 and the auxiliary amplifier 28.
The method of adding one more variable attenuator and one more variable phase shifter to each loop has a number of disadvantages such as low reliability, an increase in cost, and an increase in transmission loss because of an increase in the number of circuit parts. The method of adjusting the gains and phases of the main amplifier 18 and the auxiliary amplifier 28 contains a problem that it is difficult to make separate adjustments of the gains and phases such as change of only the phase without changing the gain or change of only the gain. Further, a skilled engineer is needed for adjustments because very flat frequency characteristics are required for the FF amplifier as described above.